1. Field of the Invention
The present invention relates to an electronic device represented by in-vehicle power module, and particularly relates to a structure of mounting an electronic component such as a semiconductor element on a substrate.
2. Description of the Related Art
From consideration for environmental issues, vehicles using electric motor drive, which are represented by a hybrid electric vehicle, are expected to be increasingly widespread in future. Moreover, an electric control unit (ECU) using the electric motor tends to be used for various units that have been controlled by hydraulic pressure such as power steering or a brake system. For ECU for motor control, an IDBT (Insulated Gate Bipolar Transistor) chip or FET (Field Effect Transistor) chip, which allows large current flow, is used, and heat of several ten watts to several hundreds watts is radiated from the chip (electronic component such as semiconductor element). Therefore, the chips or electronic devices having the chips are necessary to be cooled by a water cooling system or an air-cooling system. In an example of a sectional structure of an ECU module for motor control as shown in FIG. 1, a lower drain electrode of an FET chip 1 is connected onto an electrode 5 of a ceramic substrate 7 having good heat conductance, with solder 3, and the ceramic substrate 7 is adhered to a heat sink with a resin adhesive 11 having good heat conductance. Upper gate electrodes and source electrodes of the FET chip 1 are connected to predetermined electrodes 15 on the ceramic substrate 7 by a large number of wire bonding lines (bonding wires) 13. A structure of an electronic device requiring such large number of lines of wire bonding requires large investment in facility due to a process of manufacturing the device, in addition, manufacturing of the device itself requires long time and high cost. Furthermore, since surfaces of the electrodes 15 for the wire bonding needs to be kept clean, a cleaning step is separately necessary after the chip 1 has been connected with solder to the substrate 7.
Currently, calorific value of the FET chip mounted in an electronic device (for example, power module) tends to be increased, therefore an excellent heat radiation performance is requested to a structure of the device. Such request is made not only to electronic devices for vehicles (in-vehicle power modules) but also to other electronic devices used for control systems of inverters for railway vehicles, generators, lighting fixtures or thermal appliances and the like. If a step of the wire bonding can be eliminated in manufacturing the electronic devices, significant reduction in manufacturing cost can be estimated.
There was provided a structure of an electronic device as shown in FIG. 2 in the related art. That is, a ceramic material having high heat conductivity is used for the substrate 7, and the chip 1 is mounted on the ceramic substrate 7 with solder 5, and furthermore the ceramic substrate 7 is connected to a heat sink 19 for heat radiation with solder. A metal frame 21 formed from a material having excellent heat conductance such as Cu (copper) is used as a connection member in place of typical wire bonding lines, which is connected to electrodes on the chip and electrodes 23 on the substrate 7 with solder.
In addition, a report on the technique of solder connection is published in the following non-patent literature 1.
[Non-patent literature 1]
Ricky W. Chuang et all: 51th Electronic Components and Technology Conference (2001), pages 671-674